Tapping Long Holes

I confess that it's been 50 years since I took shop classes in college and while I've done some things regularly since then both professionally and as part of my hobby work, I've had very limited experience in certain areas. SO ...

I've got a bar of aluminum that I had to drill and tap several 1/4-20 holes in. The bar is roughly 7"x1"x1.25" and drilling and tapping the holes through the thickness (1") was a good refresher course, but the hole through the height (1.25") has me stumped. Without thinking it through, I drilled the hole and started tapping. But the tap's cutting length is only 1" and the shaft behind it is too thick to pass though previously tapped metal.

A Google search reveals that there are some custom taps that are made for extra long holes but for a one-off I don't need a custom tap. I think I found that there are some types where the shaft is narrow enough to pass through the previously tapped hole to allow deeper holes to be done, but the nomenclature is confusing.

I need help and I'd love to learn the right terminology. As a start, to get the project back on track, I'd really appreciate it if someone could tell me, perhaps, the name and part number at ENCO for the right tap for my hole. As a follow-up, perhaps a link or three to pages which explain the differences would also be greatly appreciated.

TIA Norm

Reply to
Norm Dresner
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Do you need the whole length of the hole threaded?

If not, drill a clearance hole for the tap and keep going.

If you do, you could grind the shank of the tap for the extra quarter inch; a cheap carbon tap in aluminium should be ok, especially if you have a good sized tap hole and some wd-40 with which to lube the tap.

I presume you don't have a lathe - single-pointing with a boring-bar works.

Reply to

There are several kinds of taps with extra-long shanks, called pulley taps (from their use in tapping pulley hubs), extension taps, or long reach taps.

The words you're interested in are "reduced shank" or "undercut shank". An ordinary 1/4-20 pulley tap will have a 1/4" diameter shank, while a reduced shank tap will have a shank smaller than the root diameter of the thread. So you can keep going and going ...

Spiral point (gun) taps are particularly nice here, since they throw the chips ahead of the tap and they don't clog the flutes.

In your case, see for #325-4824 -- a 6" long reduced shank 1/4-20 spiral point tap for $20.95.

I've only used mine a few times, but I'm very glad I have it.


Reply to
Tove Momerathsson

Enco doesn't like my 3rd party cookie setting so I can't show you, but "Nut Taps" are long ones with reduced shanks that fit through the tapped hole. Pulley taps have thicker, stronger shanks to get past the flange to the hub.

Grind the shank of a less valuable tap smaller beyond the threads. You could tap the end of an aluminum rod for a handle to hold it and protect the threads from overheating if it's carbon steel. It will be weakened so don't use it unless you have to.

You can make a crude quickie tap for soft metal by grinding the end of a screw conical and filing lengthwise notches or grinding a flat on either side, like the end of a chisel.

Jim Wilkins

Reply to
Jim Wilkins

| | Do you need the whole length of the hole threaded? | | If not, drill a clearance hole for the tap and keep going. | | If you do, you could grind the shank of the tap for the extra quarter inch; | a cheap carbon tap in aluminium should be ok, especially if you have a good | sized tap hole and some wd-40 with which to lube the tap. | | I presume you don't have a lathe - single-pointing with a boring-bar works.

I've thought of drilling a clearance hole for the top 1/4" or so, but it bothers my sensibilities ... I may yet do it.

The tap I'm using is part of a set I bought from DRI around 1973 or so and yes, I could probably grind the shank down but that seems like a lot more work than it's worth, considering that new taps can be bought for $5-7.

And, right, I don't have a lathe.


Reply to
Norm Dresner

only because its alum maybe try tapping as deep as you can with your tap, get a 1/4 bolt or carriage bolt about 4" long and grind tip to look like your tap or close and vise-grip and have at it.

Reply to

Actually very long tapped holes are not a good idea. The slight variation in lead between the tap and the hole will frequently cause the tap to bind/break. You are attempting to tap 5 diameters deep.

A tap depth of 1 to 2 diameters [in your case 1/4 to 1/2 inch] will be adequate. Indeed, it has been frequently demonstrated in most cases even a 1 diameter tap depth [assuming good threads and a reasonable thread percentage] will result in fastener breakage before the threads will strip when loaded to destruction.

Without knowing your actual design, it would appear best to drill a counter bore large enough to pass the tap shank, if you will be using the drill press as a tapping alignment jig. If you tap from the other side, the counterbore can be just enough to clear the tap, say 5/16 or 9/32. It can be helpful to put a heavy chamfer [or counterbore] on the side where the thread exits/enters as this will keep the surface smooth/level, as you will tend to raise a bur where the trap enters/exits the material. If you do this remember to take the chamfer depth into consideration when calculating the length of tapped hole.

Although soft, aluminum can be tricky to tap, so back off often to break the chips, back out often and clean if the chips are tending to pack in the flutes, use a tap wrench [a crescent wrench is a sure way to break a tap] and use a good tap lube, WD-40 if nothing else.

Good luck on the project, and let the group know how you make out.

Unka' George [George McDuffee]

------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).

Reply to
F. George McDuffee

I thought of a variation on that during lunch, tried it, and it worked pretty well. A home-made tool will probably cut with only one edge anyway, so why make more?

I hand ground a bevel about 2/3 of the way across the end of a Grade 5

1/4-20 screw, trying for a 30 degree angle (it measures 33). After filing the burrs off I used it to complete a hole tapped 1/4" deep in 3/4" aluminum, with Tap-Free lube. It acted like a spiral point tap should and spit out one long curled chip. The exit wound is tighter than standard but another screw will go in with finger pressure.

Jim Wilkins

Reply to
Jim Wilkins

good advice


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Reply to
Gunner Asch

Nut tap.

McMaster cat# 2583A12 Nut Hand Taps 1/4"-20, 4 Flute, 5" O'all Length In stock at $40.95 Each

Or for a lot less than $40 you can grind down the shank of a standard tap by hand. It won't be pretty, but it'll work.

Reply to
Ned Simmons

Reply to
Jim Wilkins

Thanks to everyone who answered. I've learned the term "Pulley Tap" and while it might have been the ideal thing, I took the easy way suggested by several people and ground down the shaft of the tap enough to clear the threads for the final 1/8" or so.

Thanks again Norm

Reply to
Norm Dresner

Do you have a spin indexer? A surface grinder or a Tool & Cutter grinder? Then grind the diameter of the shank down a little below the root diameter -- and perhaps overlapping the last turn of threads, for a length a bit over 0.250" removed from the shank,

Be careful with this, because it will probably be easier to break the shank off in the workpiece.

Look for a "reduced shank" (or something like that).

Good Luck, DoN.

Reply to
DoN. Nichols

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